Inertial navigational systems generally include gyroscopes, such as ring laser gyroscopes, to sense rotation about different axes. In the ring laser gyroscope, the HeNe glow discharge of the laser tube is controlled by a high voltage power supply operating in conjunction with a current regulator.
As is known in the art, a common cathode of the laser tube operates with two anodes whose current must be regulated to control the glow discharge and to maintain constant tube intensity. The current regulator typically is implemented with an operational amplifier feedback loop and a transistor.
FIG. 1 illustrates a typical current regulator design for use with a gas laser tube. As shown, a power supply 10 generates a high voltage to operate a laser tube 12. The high voltage is applied to the tube's cathode 12c. The tube current is controlled by identical current regulators coupled to the tube's anodes 12a. Each current regulator includes a ballast resistor 14, an operational amplifier 16, a transistor 18, a current sense resistor 20, and a reference voltage Vref 22. Generally, each transistor 18 is a P-channel fet with a voltage rating of 450 VDC.
During operation, for each current regulator, operational amplifier 16 drives transistor 18 to maintain a constant voltage drop across current sense resistor 20. The current is controlled to a value of the reference voltage Vref divided by the value of current sense resistor 20. In a typical operation, transistor 18 is biased to have a 100 to 200 voltage across it, which generally requires that power supply 10 be tightly regulated to account for voltage variations occurring as a function of temperature within laser tube 12. Voltage variations also occur throughout the natural life of laser tube 12.
Moreover, to protect each transistor 18 from the high voltages that are required for the anode, each current regulator is provided with protection diodes 24. Unfortunately, these protection diodes place a severe limitation on the dynamic range of the current regulator, typically limiting operation to approximately 320 VDC.
This limitation, in turn, forces tight regulation requirements on the power supply design, and requires large values for the ballast resistor 14. Moreover, the design requires that the value of the ballast resistor 14 change for different current levels. These problems are particularly acute due to the substantial difference between the start-up voltage (e.g., 3000 VDC) and the operating-run voltage (e.g., 1000 VDC) applied to the laser tube's cathode required with typical gyroscopes.
It is therefore an object of the present invention to provide a current regulator that has a wide dynamic range and large operating voltage capability, that uses a lower value ballast resistor, and that requires lower power supply voltage regulation.
Various other objects, advantages and features of the present invention will become readily apparent to those of ordinary skill in the art, and the novel features will be particularly pointed out in the appended claims.